cmd/ld: fix another unsigned value causing bugs on Plan 9 "The usual conversions" bite again.
cmd/ld: fix another unsigned value causing bugs on Plan 9
"The usual conversions" bite again.

R=golang-dev, rsc
CC=golang-dev
https://codereview.appspot.com/9103044

h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang,h8liu/golang